1. Field of the Invention
The invention provides an improved molded thermoplastic closure, especially for containers of pressurized fluid, such as carbonated beverages.
2. Description of the Prior Art
The typical prior art thermoplastic closure comprises a flat disc-shaped panel and an annular skirt depending from the periphery thereof, threaded to engage corresponding threads on the container finish. A seal is formed by an annular plug depending from the flat panel, constructed and arranged to engage the inside annular surface of the finish by an interference fit. One example of such a prior art closure is shown in U.S. Pat. No. 4,016,996 to Aichinger et al.
A problem common to all threaded closures for pressurized fluid is the possibility of the premature release of the closure from the container finish during removal of the closure. As the closure is unthreaded, the seal is broken, and pressure within the head space is applied to the panel and the skirt of the closure, tending to deform the skirt and disengage the threads. If either the container threads or the closure threads are not properly formed, the closure can be prematurely stripped from the threads. While some prior art closures provide means for venting the head space as soon as the seal is broken by axial movement of the cap, the pressure must be vented while there is sufficient thread engagement to retain the closure. This problem tends to be greater in closures which have a plug inserted within the finish, because the closure must be unthreaded a significant distance before the plug is removed from the finish, thereby breaking the seal. The reduced thread engagement when the seal is first broken can increase the possibility of premature release.
Although careful thread design can alleviate the problem by permitting venting of head space through the threads, various other configurations of vents in the threaded finish and in the closure itself have been suggested. For example, U.S. Pat. No. 4,007,848 to Snyder discloses a container finish having one or more axial grooves through the threads. U.S. Pat. No. 4,007,851 to Walker discloses a metal closure having radial venting ports formed in the skirt.
A second problem with the typical thermoplastic closure is seal failure, especially in conditions of elevated temperatures and pressures. Any thermoplastic material will experience some degree of cold flow which will adversely affect seal performance. Therefore, closures are designed to be "self sealing," that is, increased internal pressure is utilized to enhance sealing performance.
A thermoplastic closure including an annular plug seal is designed to be self sealing in this manner. Ideally, increased internal pressure acting on the inside annular surface of the plug increases the outward force of the plug against the inside annular surface of the finish, thereby enhancing seal performance. In practice, however, this effect is very minimal, in part because of the short axial extent of the plug, which is therefore rather rigid. In the typical closure design, the axial extent of the plug is limited by the requirement that the head space be vented early in the removal operation, while there remains sufficient thread engagement to retain the closure.
Because the self sealing effect in the typical annular plug closure is minimal, such closures rely primarily on the initial interference fit between the plug and the finish to effect a seal. The relatively large interference thus required undesirably increases the closure removal torque requirement. Another approach is to provide the lower edge of the plug seal with a deformable lip or skirt to improve the seal, such as is shown in U.S. Pat. No. 4,090,631 to Grussen. This approach requires a more complex mold design and makes resealing difficult.
Finally, a major problem in the prior art is the loss of the seal caused by distortion of the closure due to internal pressure. In the common flat paneled closure, high internal pressures will tend to cause the panel to bulge outwardly. This distortion of the panel moves the plug upwardly along the container finish, thereby moving the location of the seal. It is known that such a dynamic seal is significantly more subject to failure than a static seal and that such distortion necessarily requires the plug seal to extend axially well into the bottle finish, thus increasing venting problems when the cap is removed as discussed above.